Attendances over the game are far different to previous decades and been steadily increasing since the mid 80s
To think that Leeds couldn't fill it after being extended is being naive and it's obvious the owners will have looked into it in detail. With the current 35k crowds, they get £30m in gate receipts as they limit season tickets and don't have generous concession prices. To put that into perspective, we got £11.6m from 40k fans so there's a huge difference in what fans pay on average at both clubs. They're actually 7th highest paying fans if you just do the basic calculation of match day revenue divided by fans as I hoyed this up previously and was surprised at it.
These were their prices for 2018/19 after the finished 13th and had been out of the Premier League since 2003, spent 3 seasons in League One in 2007-2010 then spent 6 of the following 8 seasons finishing 13th or below in the Championship and just average over 31k in 2017/18. In many areas it's more expensive than we are now.
If we had exactly the same pricing structure as Leeds limiting ST sales and concession areas then we wouldn't be getting the 40k we get now. That's what's class about the SOL size as the owners can't take the piss with pricing due to demand exceeding supply. Though as we've seen this season they've dramatically increased concession prices in the East & West stands and dropped the U16 down to U14. This is only the start as it will change more if we hit the Premier League as they know crowds will increase.
Leeds will easily get 1,000s more attending and they've got plenty of wiggle room to create concessions that would attract parents to take kids to games like we can. This then gets them hooked at an early age and gets them to go to games even when times are crap.
All this discussion about crowds online is usually just pointless dick waving as there's far more than just looking an average number, unless it's being used in a school playground between kids.